1. Field of the Invention
The present invention relates to a system for processing image data for a laser pattern generator.
2. Description of the Prior Art
In the U.S. Pat. No. 4,541,712 entitled "Laser Pattern Generating System"assigned to TRE Semiconductor Equipment Corporation, the assignee of the present case, there is described a system useful for producing photolithographic masks or directly exposing image patterns on a semiconductor wafer or other target. A portion of that pattern generator 10 is shown in FIG. 1 herein.
To generate a pattern on a semiconductor substrate or other target 11, the beam from a laser 12 is split into a set of sixteen spaced parallel beams 13 by a splitter array 14. The set of beams 13 is repetitively deflected in the y-direction across a region 15' of the target 11 by a deflector 16 driven by appropriate electronics 16'. Advantageously, the target 11 is moved along the x-direction so that during consecutive deflections the set of beams 13 will scan across adjacent regions or blocks 15 of the target 11. In this manner an entire stripe or swath 17 of the target 11 will be covered block-by-block during successive deflections of the beam set 13. Thereafter, adjacent swaths 17' may be exposed in like manner by first appropriately moving the target 11 in the y-direction and then repeating the successive beam deflection operations.
Advantageously, each beam in the set 13 impinges on the target 11 at a different but adjacent position along the x-axis. (To avoid mutual interference, adjacent beams may be offset in the y-axis as illustrated at 18 in FIG. 1 and described in the above-identified copending application.) With such arrangement, each block 15 has a width of sixteen picture element ("pixel") positions along the x-axis. As the deflector 16 sweeps the beam set across a block 15, each beam crosses a set of consecutive picture element positions along the y-axis. In the illustrative system described herein, there are 1024 such y-axis locations in each block 15.
To form an image on the target 11, each beam in the set 13 is individually modulated or appropriately turned on and off by a corresponding one of a set of modulators 19. This modulation occurs while the beams are being deflected across each block 15.
FIG. 2 shows a typical pattern 21 which may be generated on a target 11 by the generator 10. The shaded areas represent regions where the beams have been turned on, for example, to expose a photosensitive coating on a mask used for integrated circuit production, or to expose such a coating situated directly on a semiconductor wafer. The shaded regions in the patterns 21 thus may represent e.g., regions where an oxide layer is to be removed from a semiconductor wafer to provide defined openings for the diffusion of dopants into a semiconductor substrate. However, the invention is not limited to such applications.
In FIG. 2, the generated pattern 21 is shown overlying a grid 22 which defines each of the actual blocks 15 that are swept by the set of beams 13. Of course, such grid would not actually appear on the target surface, and for that reason, it is drawn in phantom in FIG. 2. Note that the grid 22 is not rectilinear, but rather that the sides of the blocks in the swaths 17a through 17g are angled or skewed with respect to the y-axis. This results since, in the preferred embodiment of the generator 10, the x-direction movement of the target 11 takes place continuously, during the actual deflection of the beams 13. Therefore, the loci of points swept by the beam 13 during each deflection defines not a rectangle, but rather a non-rectangular parallelogram. Accordingly, one of the objectives of the inventive data handling system is to provide image data to the modulators 19 in a format which compensates for the block skewing that results from the lateral translation of the target 11 during the beam sweep.
Note also in FIG. 2 that alternate swaths are skewed in opposite directions. This results because, to minimize the time required to generate an entire pattern, alternate swaths are processed in opposite directions. In other words, during production of the swath 17a the target is moved from right to left, so that the beams 13 sweep successive blocks in the +x direction. During production of the next swath 17b the target is moved toward the right, so that successive blocks are swept in the -x direction. The resultant order of block scanning is indicated by the arrow 20. Another object of the present invention is to provide image data to the modulators 19 which takes into account the opposite direction of target travel on alternate swaths.
Another objective of the present invention is to provide a system for converting image data into a bit map format that can be used directly for controlling the modulators 19. To accomplish such control, the data must be available in real time, in the form of turn-on and turn-off commands to the modulators 19. That is, as the deflector 16 sweeps the beams 13 across a block 15, signals must be provided to the modulators 19 to turn on and turn off each individual beam during the deflection so as to create the desired pattern.
Consider, for example, the section 21a of the pattern 21 (FIG. 2) which consists of two overlapping rectangles 23 and 24. The upper left corner of the rectangle 23 lies within a block 15a. An enlarged but unskewed corresponding block 15a' is shown in FIG. 3A. The loci of the beams 13-4 and 13-5 (assuming no movement of the target 11) also are shown. Note that as these beams are swept downward, the beam 13-5 first must be turned on when it reaches the position 25a. Slightly later, the beam 13-4 is turned on when it reaches the position 26a. Later, the beam 13-4 and 13-5 are turned off in that order when they reach the points 26b and 25b respectively.
In typical design systems for laying out large scale integrated circuits, the information defining each graphic element in the LSI layout normally is not provided in a form which can be used directly for the real time control of a laser pattern generator. More typically, the information is provided in the form of a data list specifying each rectangular element in the pattern. FIG. 4 illustrates this for the pattern members 23 and 24. Each is specified as a rectangle having a certain length L, width W, center position (x, y) with respect to an arbitrary rectangular coordinate system, and an angle .alpha. specifying the angle between the longitudinal center line of the rectangle and the x-axis of the coordinate system. FIG. 4 shows typical numerical values (in arbitrary units) for the rectangular members 23 and 24 of the pattern 21.
This rectangle-format LSI pattern design data must be converted into a format for direct control of the pattern generator modulators 19, and this is another objective of the present invention.
An advantageous format for the real time control data for the modulators 19 is that of a bit map. This is a set of control data stored in a memory having a designated storage location corresponding to each of the pixels in the block being scanned. For the generator 10, such a bit map memory may contain 1024 storage positions (one for each vertical dot position) for each of the sixteen beams, for a total of 16,384 locations. The information from such bit map memory then may be supplied to the modulators 19 via a buffer 27 and appropriate modulator drive electronics 28 (FIG. 1) during actual deflection of the beam set 13 by the deflector 16. Thus it is a further object of the present invention to provide an image data handling system which will convert image source data, such as the rectangle list of FIG. 4, into stored bit map data that can be used in real time to control the pattern generator modulators.
One approach to accomplishing such data conversion is to utilize an intermediate vector data format in which each side of each rectangular graphic element is represented by a vector that specifies the turn-on or turn-off direction for the modulators 19. The two edge segments 23-1 and 23-2 of the rectangle 23 which are shown in FIG. 3A may represent such vectors. It is a further object of the present invention to provide a data handling system for a pattern generator that utilizes such intermediate vector format for the image data.
Such vector formatted data itself advantageously is converted into a bit map precursor, which may be a set of turn-on and turn-off control signals that are associated with respective beams. These signals are stored in an image buffer memory at locations associated with the deflection angle or vertical pixel position at which the respective beam is to be controlled.
A fragmentary portion of such bit map precursor memory contents is shown in FIG. 3B, and corresponds to the vector information of FIG. 3A. In this precursor, the column marked x.sub.b4 represents the set of memory locations associated with the beam 13-4, while the column x.sub.b5 is associated with the beam 13-5. The numbers along the ordinate represent memory locations corresponding to vertical pixel positions in the block 15. For example, the memory location 800 would correspond to the vertical position 800 which is about eight-tenths of the way between the top and the bottom of a block 15. For the beam 13-4, stored in the memory position 760 (for the column x.sub.b4) is an "on" command indicating that the beam 13-4 is to be turned on at the corresponding block location or deflection angle, as indicated at the point 26a in FIG. 3A. A turn-off command is stored at location 870, corresponding to turn-off of the beam 13-4 at the point 26b.
The precursor memory contents in FIG. 3B do not take into account either the overlap of two rectangular image members (such as in the region 23' of FIGS. 2 and 4) or the skewing required by the lateral translation of the target 11 during pattern generation. However, it is a further object of the present invention to provide a data handling system which converts image data from a vector format to a bit map precursor format, and which also converts such precursor data into bit map information that does take into account both pattern element overlap and skewing due to lateral target movement.
Another consideration is the large amount of information which must be handled to create a typical large scale integrated circuit pattern. With present day technology, the pattern utilized during one processing step in the fabrication of an LSI may have some 70,000 quadrilateral members such as those exemplified by the rectangles 23 and 24. This may convert into a pixel matrix exceeding one billion bits. Current storage technology does not permit the economical storage and real time access of the entire one megabit or larger pixel image data. Therefore it is necessary to store the image data in a more compact format, and to generate the bit map in pieces, in real time synchronism with the repetitive beam sweep operations of the pattern generator.
Thus it is another object of the present invention to provide a data handling system for a pattern generator that can translate an extremely large number of quadrilaterals into a bit map which is created piecewise at a rate commensurate with the real time modulation data demands of the pattern generator.